<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .father{
            position: absolute;
            top: 0;
            left: 0;
            width: 100px;
            height: 600px;
            background-color: gray;
            z-index: 10;

        }
        .son{
            position: absolute;
            top: 0;
            left: 0;
            width: 50px;
            height: 50px;
            background-color: red;
            z-index: 1;
        }
    </style>
</head>
<body>
<div class="father">
</div>
<div class="son"></div>
</body>
</html>